One or more fixed guide rails are usually situated at the upper end of the base of a machine tool, so that a movable carriage, which supports the machine tool, such as a grinding wheel or belt grinder, can be advanced, relative to the workpiece. A motor drives a lead screw assembly secured to the movable carriage so that the movable carriage is advanced relative to the workpiece. The workpiece is retained in fixed position between a head stock and a foot stock. The movable carriage, and the machine tool mounted thereon, are movable perpendicular to the centerline of the workpiece.
The movable carriage is usually a casting, and the base is a separate casting of considerably greater weight and size to provide stability for the machine tool. The guide rails are made of high-grade machine steel that has been manufactured, and finished, to exacting, extremely precise tolerances. Each guide rail includes two parallel surfaces, extending upwardly from the base. The underside of the movable carriage is configured to straddle the fixed guide rails.
Hydrostatic bearings are situated on opposite sides of the fixed guide rails to provide a thin film of fluid, such as oil, in the gap between the relatively movable parts, to allow the movable carriage to glide smoothly therealong. The film of fluid is introduced into the bearings, under pressure, from a remote source. Pressure is maintained in the bearings by the small gap between the adjacent surfaces. The gap may be in the order of a few thousandths of an inch.
The guide rails have been machined, in recent years, to ever tighter tolerances, as the demand for more accurately ground workpieces, and higher production rates, has increased significantly. The machining of both faces of the guide rail in a parallel, true fashion, has become an functional necessity; the machining operation is complicated by the length of the guide rail.
The underside of the movable carriage straddles each guide rail, and the facing surfaces of the movable carriage are designed to be spaced from the parallel, and true, faces of the guide rail. The bearings, usually located in the movable carriage, attempt to retain the movable carriage properly centered with respect to the guide rail, so that the machine tool can be advanced into working relationship with the workpiece, in an efficient, error-free manner. If the faces of the guide rail are not true and parallel, the movable carriage may shift a few thousandths of an inch closer to one facing surface, or the other, thereby reducing the accuracy of the machine tool. In extreme cases, if the faces are not parallel and true over their extended length, the gap may be diminished significantly, or even eliminated, so that the opposing metal surfaces of the movable carriage and the guide rail may come into contact. The movable carriage will then be seized, or "frozen", relative to the guide rail, with attendant scarring and structural damage to the contacting surfaces.
In order to maintain the movable carriage centered relative to the guide rail, fixed hydrostatic bearings were located in the underside of the movable carriage. The fixed hydrostatic bearings received a supply of pressurized fluid, through an aperture, or port, that discharged the fluid into the space or gap between the underside of the movable carriage and the opposing face of the guide rail.
Fixed hydrostatic bearings proved to be satisfactory when operating tolerance were fairly generous. However, as the operating tolerances were tightened in recent years, the fixed hydrostatic bearings have proven to be very difficult, and costly, to manufacture to the appropriate tolerances.
One proposed solution suggested the introduction of a pre-load mechanism into the movable carriage of the machine tool; such pre-load mechanism functioned in opposition to the conventional hydrostatic bearings. The pre-load mechanism introduced hydraulic fluid, such as pressurized oil, into the gap between the movable carriage and the guide rail. Hence, the pre-load mechanism tended to stiffen the sliding components of the machine tool and resist deflection.
To illustrate, U.S. Pat. No. 3,619,013, granted to Gordon H. Jones, discloses a hydrostatic way-bearing construction utilizing hydraulic pre-loading of opposed way bearing surfaces (18, 28; 19, 29) of the ways (15, 16) between relatively slideable machine tool members (table 11 slides on saddle 12). The construction comprises pre-load gibs (42; 54) having hydrostatic bearing surfaces (28, 29; 34). The pre-loading is accomplished by a plurality of longitudinally spaced hydraulic actuators (46 operatively associated with horizontal gib 42; 59 operatively associated with vertical gib 54). Some of the hydraulic actuators (46) are recessed within the holddown cleats (cleats or clamp plates 22, 24), while other actuators (59) are recessed within one of the vertically oriented pre-load gibs (such as gib 54, as shown in FIG. 5) of the Jones patent.
Preload gib (42) of U.S. Pat. No. 3,619,013 is formed as a flat bar extending substantially the length of the table (11), and is housed within a corresponding recess (44) in cleat plate (22), as noted in column 2, lines 53-44. A plurality of pre-load hydraulic actuators (46) are disposed along the bottom of recess (44), and pressurized oil or other hydraulic fluid is supplied to the hydraulic cylinders (48) for the actuators (46). The pre-load gib (42) thus provides a constant pre-load over the length of the table (11). The vertical pre-load gib (54) functions in a similar manner.
The "floating" gibs are movable toward, and away, from the guide rail on the saddle 12 in the Jones patent, to accommodate minor surface irregularities, or imperfections, within the face of the guide rail. Whereas the foregoing pre-loaded hydrostatic bearing system shown in U.S. Pat. No. 3,619,013, represented a marked improvement over fixed bearing systems, systems such as shown in Jones, require the manufacture and installation, of gibs, which are lengthy, rigid flat bars, that do not readily accommodate localized imperfections. Gibs (42) are restrained in movement longitudinally, within recess (44), by end retainer plates (45; see FIGS. 4 and 8), and are restrained laterally by the walls of the recess. Similar restraints are imposed upon the vertical gibs (54).
Despite advances in hydrostatic bearing systems, machine failures, wherein the movable carriage contacted or engaged, the guide rail, continued to occur. Such failures, whatever their cause, would have catastrophic consequences, for production lines would be shut-down for time consuming and costly repair, with attendant delays in production of product.